Switching power supplies are a preferred embodiment in many power processing applications because of their high efficiency and theoretical power density advantage (i.e., a small unit can theoretically process large amounts of power). Since power processing is achieved by the use of switching devices alternately transitioning between conducting and nonconducting states in an interval of time less than a microsecond, high radio frequency harmonic signals are generated. These radio frequency signals flow in a plurality of networks of circuit paths or closed circuit loops, both within and without the actual physical domains of the switching power supply. These circuit paths include the actual conductors apparent in the circuit schematic, and more significantly circuit paths connected by rapidly charging and discharging actual and parasitic capacitances within the power supply which include filter capacitors, the power transistor to heat sink capacitance, transformer winding capacitances, wiring to ground capacitances, and a host of other sources of parasitic capacitances. These circuit paths also include input and output access leads which act as antennas and radiate electromagnetic energy in response to these circulating RF currents. Not only does this radiated energy interfere with other electrical equipment, but it and the RF currents are required to be maintained below certain limits by statutory and regulatory standards such as the FCC Part 15, VDE, IEC and similar standards.
The most common technique of suppressing these RF currents and radiated emissions (RFI) is to place low pass filters on the input and output leads of the power supply in order to attenuate these currents. Since the noise is radiated into all the leads of the power supply, these filters must encompass every individual lead coming into and going out of the power supply, and must operate to attenuate both normal and common mode noise currents. These filter circuits unfortunately add considerable size and weight and expense to the overall power supply circuit package and also reduce its overall efficiency. Stated concisely, the filter circuits nullify many of the advantages for which a switching power supply was selected in the first place, and for all that, they do not provide a complete solution to the leakage current problems because the filter capacitors also conduct line frequency currents, thus introducing dangerous leakage currents into the equipment grounding wire whose magnitude may exceed limits allowed by codified safety standards such as UL, IEC, VDE, etc.